Positive active material for rechargeable lithium battery and method of preparing same

ABSTRACT

Disclosed is a positive active material for a rechargeable lithium battery. The positive active material includes at least one compound represented by formulas 1 to 4 andl a metal oxide or composite metal oxide layer formed on the compound.  
                                           Li x Ni 1−y Mn y F 2     (1)         Li x Ni 1−y Mn y S 2     (2)         Li x Ni 1−y−z Mn y M z O 2−a F a     (3)         Li x Ni 1−y−z Mn y M z O 2−a S a     (4)                                 
 
     (where M is selected from the group consisting of Co, Mg, Fe, Sr, Ti, B, Si, Ga, Al, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, U, Np, IPu, Am, Cm, Bk, Cf, Es, Fm, Md, No and Lr, 0.95≦x≦1.1, 0≦y≦0.99, 0≦,z≦0.5, and 0≦a≦0.5)

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is based on application Nos. 00-12504 and01-4898 filed in the Korean Industrial Property Office on Mar. 13, 2000and Feb. 1, 2001, the content of which is incorporated hereinto byreference.

BACKGROUND OF THE INVENTION

[0002] (a) Field of the Invention

[0003] The present invention relates to a positive active material for arechargeable lithium battery and a method of preparing the same, andmore particularly, to a positive active material for a rechargeablelithium battery exhibiting good electrochemical properties and a methodof preparing the same.

[0004] (b) Description of the Related Art

[0005] Rechargeable lithium batteries use a material from or into whichlithium ions are intercalated or deintercalated as positive and negativeactive materials. Rechargeable lithium batteries produce electric energyby an oxidation and reduction reaction during the intercalation anddeintercalation of lithium ions.

[0006] For the positive active material in the rechargeable lithiumbattery, chalcogenide compounds into or from which lithium ions areintercalated or deintercalated are generally used. Typical examplesinclude LiCoO₂, LiMn₂O₄, LiMnO₂, LiNiO₂, or LiNi₁XCoXO₂ (0≦X≦1). LiCoO₂provides with good electrical conductivity, high cell voltage of about3.7V, good cycle life and safety characteristics, and high dischargecapacity of 160 mAh/g, thus it is widely used. However, it is veryexpensive and the cost portion of LiCoO₂ reaches to 30% of the totalmanufacturing cost of the battery. Therefore, it is desirable to developa low cost positive active material to replace LiCoO₂.

[0007] Manganese-based materials such as LiMn₂O₄ or LiMnO₂ are easy toprepare, less costlier than LiCoO₂, and environmentally friendly, andhave higher cell voltage (3.9V) than that of LiCoO₂. However, themanganese -based materials have a low capacity of about 120 mAh/g whichis smaller than 1:0 that of LiCoO₂ by 20%. Thus, with manganese-basedmaterials it is difficult to fabricate high capacity or thin batteries.LiNiO₂ is also lower cost than LiCoO₂ and has a high charge capacity,but is difficult to produce. LiNi_(1−x)Co_(x)O₂ (0≦X ≦1) has also alarger capacity (200 mAh/g) than LiCoO₂, but lower discharge potential,inferior cycle characteristics to LiCoO₂, and poor safetycharacteristics.

SUMMARY OF THE INVENTION

[0008] It is an object of the present invention to provide a positiveactive material for a rechargeable lithium battery exhibiting goodelectrochemical characteristics, which is inexpensive.

[0009] It is another object to provide a positive active material for arechargeable lithium battery exhibiting good thermal stability.

[0010] It is still another object to provide a method of preparing thepositive active material.

[0011] These and other objects may be achieved by a positive activematerial for a rechargeable lithium battery including at least onecompound represented by formulas 1 to 4, and a metal oxide or compositemetal oxide layer formed on the compound. Li_(x)Ni_(1−y)Mn_(y)F₂ (1)Li_(x)Ni_(1−y)Mn_(y)S₂ (2) Li_(x)Ni_(1−y−z)Mn_(y)M_(z)O_(2−a)F_(a) (3)Li_(x)Ni_(1−y−z)Mn_(y)M_(z)O_(2−a)S_(a) (4)

[0012] (where M is selected from the group consisting of Co, Mg, Fe, Sr,Ti, B, Si, Ga, Al, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho,Er, Tm, Yb, Lu, Ac, Th, Pa, U, Np, IPu, Am, Cm, Bk, Cf, Es, Fm, Md, Noand Lr, 0.95≦x≦1.1, 0≦y≦0.99, 0≦z≦0.5, and 0≦a≦0.5)

[0013] In order to achieve the objects, the present invention provides amethod of preparing the positive active material for a rechargeablelithium battery. In the method, at least one compound represented byformulas 1 to 4 is prepared and the compound is coated with a metalalkoxide solution, an organic solution of metal salt or an aqueoussolution of metal salt. The coated compound is then heat-treated. Thecompound represented by formulas 1 to 4 is prepared by co-precipitatinga nickel salt and a manganese salt to prepare a nickel manganese salt,mixing the nickel manganese salt with a lithium salt, and thenheat-treaeting the mixture. In the co-precipitation step, a fluorine orsulfur salt may be further used. Alternatively, a salt of metal isfurther used in the mixing step.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] A more complete appreciation of the invention, and many of theattendant advantages thereof, will be readily apparent as the samebecomes better understood by reference to the following detaileddescription when considered in conjunction with the accompanyingdrawings, wherein:

[0015]FIG. 1a is a SEM photograph showing a positive active material ofthe present invention;

[0016]FIG. 1b is an enlarged SEM photograph by 10 times of FIG. 1;

[0017]FIG. 2 is a graph illustrating the low-rate charge and dischargecharacteristics at the first cycle of positive active materialsaccording to Example and Comparative Example of the present invention;

[0018]FIG. 3 is a graph illustrating the discharge potential at thefirst cycle of positive materials according to Example and ComparativeExample of the present invention;

[0019]FIG. 4 is a graph illustrating cycle life characteristics ofpositive active materials according to Example and Comparative Exampleof the present invention;

[0020]FIG. 5 is a graph showing an XRD (X-ray diffraction) result ofpositive active materials according to Example and Comparative Exampleof the present invention;

[0021]FIG. 6 is al graph illustrating a DSC (differential scanningcalorimetry) result of a positive active material according to Exampleand Comparative Example of the present invention;

[0022]FIG. 7 is a graph illustrating the high-rate charge and dischargecycle characteristics at the first cycle of positive active materialsaccording to Example and Comparative Example of the present invention;and

[0023]FIG. 8 is a graph illustrating the charge and dischargecharacteristics after 50 cycles of positive active materials accordingto Example and Comparative Example of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] A positive active material of the present invention is aLiNiMnO₂-based material to replace LiCoO₂ which exhibits goodelectrochemical properties but is p~iQ of high cost. The LiNiMnO₂-basedmaterial of the present invention uses Ni and Mn which is lower costthan Co. It has the advantages of both LiNiO₂, which has high dischargecapacity and is low cost, and LiMnO₄ which exhibits high cell voltageand is also of low cost. In addition, the positive active material ofthe present invention has a metal oxide or composite metaloxide-included layer on a surface thereof to improve charge-dischargecharacteristics. Accordingly, the positive active material of thepresent invention has comparable electrochemical properties to that ofLiCoO₂ while it is significantly lower cost than LiCoO₂. The positiveactive material of the present invention can economically providerechargeable lithium batteries exhibiting good electrochemicalproperties (especially cycle life, high-rate characteristics, highdischarge potential, and thermal stability).

[0025] The positive active material of the present invention includes atleast one compound represented by formulas 1 to 4.Li_(x)Ni_(1−y)Mn_(y)F₂ (1) Li_(x)Ni_(1−y)Mn_(y)S₂ (2)Li_(x)Ni_(1−y−z)Mn_(y)M_(z)O_(2−a)F_(a) (3)Li_(x)Ni_(1−y−z)Mn_(y)M_(z)O_(2−a)S_(a) (4)

[0026] (where M is selected from the group consisting of Co, Mg, Fe, Sr,Ti, B, Si, Ga, Al, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho,Er, Tm, Yb, Lu, Ac, Th, Pa, U, Np, FPu, Am, Cm, Bk, Cf, Es, Fm, Md, Noand Lr, 0.95≦x≦1.1, 0≦y≦0.99, 0≦z≦0.5, and 0≦a≦0.5)

[0027] The metal in the metal oxide or composite metal oxide layer isselected from Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B or As. Thecomposite metal oxide is formed by reacting a metal salt or salts withthe metal compound represented by formulas 1 to 4.

[0028] It is preferable that the thickness of the metal oxide orcomposite metal oxide layer is 1 to 100 nm, and preferably 1 to 50 nm.If the thickness of the oxide layer is less than 1 nm, the effectobtained by coating metal oxide or composite metal oxide onto thecompound is not evident. Whereas, if the thickness thereof is more than100 nm, the oxide layer becomes undesirably thick so that the movementof lithium ions is hindered significantly.

[0029] A method of preparing a positive active material will beillustrated in more detail.

[0030] A nickel salt and a manganese salt are co-precipitated to producea nickel manganese salt. Alternatively, a fluorine salt or a sulfur saltmay be co-precipitated together with the nickel and manganese salts. Anynickel, manganese, fluorine and sulfur salts may be employed as long asthe resultant compound is capable of intercalating and deintercalatinglithium ions. However, the exemplary of the nickel salt may be nickelhydroxide, nickel nitrate or nickel acetate, and the manganese salt maybe manganese acetate or manganese dioxide. The fluorine salt may bemanganese fluoride or lithium fluoride and the sulfur salt may bemanganese sulfide or lithium sulfide.

[0031] The nickel manganese salt is mixed with a lithium salt. Thelithium salt may be lithium nitrate, lithium acetate or lithiumhydroxide, but it is not limited thereto. Alternatively, an additionalmetal salt may be added to the mixture. The metal may be Co, Mg, Fe, Sr,Ti, B, Si, Ga, Al, Sc, Y, lanthanide series or actinide series. Thelanthanide series includes La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho,Er, Tm, Yb and Lu, and the actinide series includes Ac, Th, Pa, U, Np,Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No and Lw. The example form of the metalsalt may be an oxide, a nitrate, an acetate or a hydroxide of the metal.The mixing step may be with a dry mixing process, or a wet mixingprocess using organic solvent as the mixing medium. The organic solventmay be an alcohol such as ethanol, or acetone.

[0032] In the above preparation, a compound represented by one offormulas 1 to 4 is obtained.

[0033] The mixture is heat-treated (first heat-treatment) in a stream ofair to prepare a compound represented by one of formulas 1 to 4. Theheat-treating step is performed at 200 to 900° C. for 1 to 20 hours inthe presence of oxygen. If the heat-treating step is performed at lessthan 200° C., the lithium salts do not react completely with the metalsalts. If the heat-treating step is performed above 900° C., Li ispartially evaporated resulting in formation of a lithium-deficientcompound. If the heat-treating is performed for less than 1 hour, thedesired crystalline material is not formed. If the heat-treating isperformed for a period longer than 20 hours, an overly crystallizedproduct is obtained or Li is partially evaporated thereby causing anunstable structure.

[0034] Subsequently, the resulting compounds are coated with a metalalkoxide solution, an organic solution of metal salt, or an aqueoussolution of metal salt. The coating process may be performed by asputtering method, a chemical vapor deposition (CVD) method, animpregnation method such as dip coating, or by using any othergeneral-purpose coating technique. Any other coating techniques, ifavailable and applicable, may be as effective as the methods describedherein. A common method of the coating process is dipping the powder inthe solution.

[0035] The metal in the metal alkoxide solution, the organic solution ofmetal salt, or an aqueous solution of metal salt may be any metal iscapable of dissolving in alcohol, organic solvents or water. Theexemplary of the metal may be Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge,Ga, B or As. A preferable metal is Al.

[0036] The metal alkoxide solution is prepared by the reaction of analcohol with metal of 0.1 to 20% by weight, preferably 0.1 to 10% byweight of the alcohol. Alternatively, the metal alkoxide is prepared bydissolving metal alkoxide. The alcohol may be methanol, ethanol, orisopropanol. The organic solution of metal salt is prepared by mixingorganic solvent with metal salt of 0.1 to 20% by weight, preferably 0.1to 10% by weight of the organic solvent. Useful organic solvent ishexane, chloroform, tetrahydrofuran, ether, methylene chloride oracetone. The metal aqueous solution is prepared by mixing water withmetal or metal oxide of 0.1 to to 20% by weight, preferably 0.1 to 10%by weight of water.

[0037] The exemplary of the metal alkoxide solution istetraorthosilicate purchased from Aldrich, Co., ortetraethylorthosilicate obtained from the mixture of silicate andethanol. The exemplary of the metal aqueous solution is vanadium oxideor ammonium vanadate.

[0038] When the metal is less than 0.1% by weight of alcohol, organicsolvent or water, the effect obtained by coating the solution onto thepowder is not evident. In contrast, when metal is more than 20% byweight of water or alcohol, the resultant coating layer becomesundesirably thick.

[0039] The coated compound is dried under ambient atmosphere followed bya heat-treatment (second heat-treatment). The heat-treating step iscarried out at 100 to 800° C. for 5 to 20 hours. If the heat-treatingtemperature is lower than 100° C., a oxide layer is not formed on thesurface, whereas, if the heat-treating temperature is above 800° C., themetal oxide or metal salt is diffused into the crystalline structure. Ifthe heat-treating step is carried out for more than 20 hours, similarproblems may occur.

[0040] As a result of the heat-treatment, the metal alkoxide solution,metal salt organic solution, or metal salt aqueous solution is convertedinto metal oxide or composite metal oxide. In this way, a metal oxide orcomposite metal oxide-coated active material is prepared. The metaloxide formed on the surface of the compound may be derived from a singleof the metal alkoxide solution, metal salt organic solution or metalsalt aqueous solution, while the composite metal oxide formed on thesurface of the compound may be derived from composite material includingcobalt, nickel, nickel-manganese or manganese salt and metal alkoxidesolution, metal salt organic solution or metal salt aqueous solution.For example, LiCoO₂ can be coated with aluminum alkoxide sol and thenthis alikoxide-coated LiCoO₂ is heat-treated to produce a positiveactive material coated with composite metal oxide of cobalt and aluminum(Co—Al—O) and/or aluminum oxide (Al₂O₃). The heating step is preferablyperformed under dry air or oxygen to obtain a uniform crystalline activematerial.

[0041] The positive active material of the present invention obtainedfrom the above procedure has spherical form, and exhibits the comparableor greater discharge capacity than LiCoO₂. Furthermore, the cost forproducing LiCoO₂ is high because expensive Co₃O₄ is used for a startingmaterial (the cost of Co₃O₄ accounts for 70% or more of the productioncost of LiCoO₂), but the positive active material of the presentinvention is produced with low cost nickel salt and manganese salt sothat the production cost is significantly reduced. The positive activematerial of the present invention has a metal oxide or composite metaloxide layer, which results in the prevention of voltage fading near theends of discharge. Accordingly, the positive active material of thepresent invention has a significant cost merit over the popular LiCoO₂for a rechargeable lithium battery without sacrificing high capacity ofLiCoO₂.

[0042] The following examples further illustrate the present invention.

Example 1

[0043] Nickel hydroxide and manganese hydroxide were co-precipitated ina 9:1 mole ratio to prepare a nickel-manganese oxyhydroxide. LiOH wasmixed with the nickel-manganese oxyhydroxide and they were mixed in amortar.

[0044] The mixture was heat-treated (first heat-treatment) at 700° C.for 20 hours while dry air was blowing on it to prepare LiNi₀ ₉Mn₀ ₁O₂powder. The size and shape of particles of LiNi₀ ₉Mn₀ ₁O₂.were confirmedby SEM and the structure thereof was confirmed by XRD.

[0045] The LiNi₀ ₉Mn₀ O₂ powder was dipped into a 5% Al-isopropoxidesolution and shaken for about 10 minutes to coat uniformly on the LiNi₀₉Mn₀ ₁O₂ powder with the Al-isopropoxide solution. The coated powder wasdried for about 2 hours under ambient atmosphere.

[0046] The dried LiNi₀ ₉Mn_(0.1)O₂ powder was heat-treated (secondheat-treatment) at 300° C. for 10 hours while dry air was blown on it.The resultant positive active material was Al₂O₃-coated LiNi_(0.9)Mn₀₁O₂.

Example 2

[0047] A positive active material was prepared by the same procedure asin Example 1 except that nickel hydroxide was mixed with manganesehydroxide in a mole ratio of 7:3, a 10% Al-isopropoxide solution wasused, the first heat-treating step was performed at 750° C. for 12 hoursand the second heat-treating step was performed at 500° C. for 10 hours.

Example 3

[0048] A positive active material was prepared by the same procedure asin Example 1 except that nickel hydroxide was mixed with manganesehydroxide in the mole ratio of 7:3, a 10% Al-isopropoxide solution wasused, the first heat-treating step was performed at 700° C. for 12 hoursand the second heat-treating step was performed at 500° C. for 10 hours.

Example 4

[0049] A positive active material was prepared by the same procedure asin Example 1 except that nickel hydroxide was mixed with manganesehydroxide in the mole ratio of 5:5, a 1.0% Al-isopropoxide solution wasused, the first heat-treating step was performed at 650° C. for 12 hoursand the second heat-treating step was performed at 700° C. for 10 hours.

Example 5

[0050] A positive active material was prepared by the same procedure asin Example 1 except that nickel hydroxide was mixed with manganesehydroxide in the mole ratio of 1:9, a 1.0% Al-isopropoxide solution wasused, and the first heat-treating step was performed at 750° C. for 20hours.

Example 6

[0051] A positive active material was prepared by the same procedure asin Example 1 except that nickel hydroxide was mixed with manganesehydroxide in the mole ratio of 5:5, a 5.0% Al-isopropoxide solution wasused, the first heat-treating step was performed at 650° C. for 12hours, and the second heat-treating step was performed at 700° C. for 10hours.

Example 7

[0052] A positive active material was prepared by the same procedure asin Example 1 except that nickel hydroxide was mixed with manganesehydroxide in the mole ratio of 7:3, a 5.0% Mg-methoxide solution wasused, the first heat-treating step was performed at 750° C. for 12hours, and the second heat-treating step was performed at 750° C. for 10hours.

Comparative Example 1

[0053] A positive active material was prepared by the same procedure asin Example 2 except that the coating with the Al-isopropoxide solutionwas not carried out.

[0054] Each of the positive active materials according to Examples 1 to7 and Comparative Example 1 were individually mixed with a Super Pconductive carbon powder, and a polyvinylidene fluoride binder (94/3/3weight ratio) in N-methyl pyrrolidone to prepare a slurry. The slurrywas coated on an Al-foil current collector to produce a positiveelectrode. Using the positive electrode and a lithium metal reference/counter electrode, a 2016-type coin cell was fabricated. A 1 M L-iPF₆solution of ethylene carbonate and dimethyl carbonate (1/1 volume ratio)was used for an electrolyte. A microporous polyethylene film was usedfor a separator.

[0055] The SEM photograph of the positive active material according toExample 2 is shown in FIG. 1a. FIG. 1b is an expanded view of FIG. 1a 10times. As shown in FIGS. 1a and 1 b, the positive active materialaccording to Example 2 has a substantially spherical form and uniformshape with various particle sizes giving an increased packing density ofthe positive active material in the positive electrode, thereby givingimproved capacity.

[0056] To evaluate the effects of the metal oxide layer on the chargeand discharge characteristics, initial charge characteristics of thecells according to Example 2 and Comparative Example 1 were evaluated.The initial charge and discharge characteristics were measured at a 0.1°C. rate between 4.3V and io 2.75V. The results are presented in FIG. 2.As shown in FIG. 2, the discharge potential and discharge capacity ofthe cell of Example 2 (a) are higher than those of Comparative Example 1(b). These improvements are deemed to be owing to the modification ofthe surface structure, i.e., the metal oxide layer on the surface of thepositive active material. In addition, the total area under the voltagecurve for the discharge (total usable energy) of the cell of Example 1in FIG. 2 is larger than that of Comparative Example 1, showing higheravailable energy for the cell of Example 1 than that of ComparativeExample 1.

[0057] In order to show the advantageous effect of the metal oxide layeron the first discharge potential, clearly, the discharge potentialsshown in FIG. 2 are re-plotted against relative (percentage) specificdischarge capacity in FIG. 3. It is evident from FIG. 3 that thedischarge potentials at approximately 93 to 98% specific dischargecapacity of Example 2 (a) is significantly higher than those ofComparative Example 1 (b) as large as about 0.1V (about 3%) at a certainpoint.

[0058] The cycle life characteristics of the cells according to Example2 and Comparative Example 1 are presented in FIG. 4. As shown in FIG. 4,the cycle life characteristics of the cell according to Example 2 (a) isslightly better than that of Comparative Example (b).

[0059] The structural characteristic of the positive active materialaccording to Example 3 was confirmed by XRD and the result is shown inFIG. 5.

[0060] The effect of the metal oxide layer on the thermal stability wasconfirmed by DSC (differential scanning calorimetry). After cellsaccording to Example 2, Comparative Example 1 and LiNi₀ ₉Co₀ ₁Sr₀ ₀₀₂O₂(Honjo, Co.) were charged to 4.3V, DSC measurements were carried out andthe results are presented in FIG. 6 (Example 2:a; Comparative Example 1:b; LiNi_(0.9)Co_(0.1)Sr_(0.002)O₂: c). It is shown from FIG. 6 that thecell of Example 2 (a) showed smallest exothermic peak, whereas those ofComparative Example 1 (b) and LiNi_(0.9)Co_(0.1)Sr_(0.002)O₂ (c) eachshowed a larger exothermic peak than that of Example 2. When the cell ischarged, the manganese active material is converted into unstableLi_(1−x)NiMn₂O₄. The bond between metal and oxygen (Mn—O) of thiscompound is easily broken releasing oxygen. The released oxygen reactswith other cell components such as the electrolyte at elevatedtemperature producing heat, and the produced heat causes the exothermicDSC peak. A smaller exothermic peak area means that the reactivity ofthe positive active material with the electrolyte is smaller. Theobservation that the active material according to Example 2 shows asmall exothermic peak means that it has an excellent stability.

[0061] Finally, the effect of the metal oxide layer on the high rate(1C) charge and discharge characteristics were evaluated by measuringthe charge and discharge voltage curves at the first cycle and thefiftieth cycles of the cells according to Example 2 and ComparativeExample 1, respectively. The results are presented in FIGS. 7 and 8. InFIGS. 7 and 8, “a” denote Example 2 and “b” denotes Comparativeexample 1. It is evident from FIGS. 7 and 8 that the charge anddischarge potentials at the first as well as fiftieth cycles of the cellof Example 1 are higher than those of Comparative Example 1.

[0062] The positive active material of the present invention have muchreduced raw material cost and therefore can be produced at a reducedcost in comparison with popular LiCoO₂ while it exhibits goodelectrochemical properties. Therefore, a high cost LiCoO₂ may be replaceby the positive active materials of the present invention. It is alsoexpected that a battery using the positive active material of thepresent invention will exhibit improved energy (Wh), cycle life, andthermal stability which is closely related to the battery safety.

[0063] While the present invention has been described in detail withreference to the preferred embodiments, those skilled in the art willappreciate that various modifications and substitutions can be madethereto without departing from the spirit and scope of the presentinvention as set forth in the appended claims.

What is claimed is:
 1. A positive active material for a rechargeablelithium battery comprising: at least one compound selected from thegroup consisting of formulas 1 to 4; and a metal oxide or compositemetal oxide layer formed on the compound. Li_(x)Ni_(1−y)Mn_(y)F₂ (1)Li_(x)Ni_(1−y)Mn_(y)S₂ (2) Li_(x)Ni_(1−y−z)Mn_(y)M_(z)O_(2−a)F_(a) (3)Li_(x)Ni_(1−y−z)Mn_(y)M_(z)O_(2−a)S_(a) (4)

(where M is selected from the group consisting of Co, Mg, Fe, Sr, Ti, B,Si, Ga, Al, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,Yb, Lu, Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No and Lr,0.95≦x≦1.1, 0≦y≦0.99, 0≦;z≦0.5, and 0≦a≦0.5)
 2. The positive activematerial for a rechargeable lithium battery of claim 1 wherein a metalin the metal oxide or composite metal oxide is selected from the groupconsisting of Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B and As. 3.The positive active material for a rechargeable lithium battery of claim1 wherein a thickness of the layer is 1 to 100 nm.
 4. A method ofpreparing a positive active material for a rechargeable lithium battery,the positive active material including at least one selected from thegroup consisting of formulas 1 to 4 and being coated with metal oxide orcomposite metal oxide, comprising the steps of: preparing at least onecompound selected from the group consisting of formulas 1 to 4; coatingthe compound with a metal alkoxide solution, an organic solution ofmetal salt or an aqueous solution of metal salt; and heat-treating thecoated compound. Li_(x)Ni_(1−y)Mn_(y)F₂ (1) Li_(x)Ni_(1−y)Mn_(y)S₂ (2)Li_(x)Ni_(1−y−z)Mn_(y)M_(z)O_(2−a)F_(a) (3)Li_(x)Ni_(1−y−z)Mn_(y)M_(z)O_(2−a)S_(a) (4)

(where M is selected from the group consisting of Co, Mg, Fe, Sr, Ti, B,Si, Ga, Al, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,Yb, Lu, Ac, Th, Pa, U, Np, IPu, Am, Cm, Bk, Cf, Es, Fm, Md, No and Lr,0.95≦x≦1.1, 0≦y≦0.99, 0≦z≦0.5, and 0≦a≦0.5)
 5. The method of claim 4wherein the compound selected from the group consisting of formulas 1 to4 is prepared by co-precipitating nickel salt and manganese salt toprepare a nickel manganese salt; mixing the resultant nickel manganesesalt with a lithium salt; and heat-treating the mixture.
 6. The methodof claim 5 wherein a fluorine or sulfur salt is further used in theco-preciipitating step.
 7. The method of claim 5 wherein a metal salt isfurther used in the mixing step.
 8. The method of claim 5 wherein theheating step is performed at 200 to 900° C. for 1 to 20 hours.
 9. Themethod of claim 5 wherein the heating step is performed under anoxidation atmosphere.